The invention relates to the formation of sound reproduction devices that operate according to the bending wave principle, and more particularly to devices that better utilize the surfaces provided for sound generation.
Sound reproduction devices that operate according to the bending wave principle are known in the art. Such devices are formed essentially of a sound panel and at least one drive system, wherein oscillations are induced in the sound panel when electrical audio frequency signals are supplied to the drive system(s). According to one feature of this type of sound reproduction device, a xe2x80x9cbending wave radiationxe2x80x9d is enabled above a lower limit frequency, also called critical frequency, wherein the bending waves in the plane of the respective sound panel cause the sound to be radiated in a direction that is frequency-dependent. In other words, a cross-section through a directional diagram shows a main lobe with a frequency-dependent direction. These conditions are valid for panels and absorbing panels with an infinite surface area. However, the conditions applying to multi-resonance panels (also refer to as distributed mode loudspeaker) which are the subject matter of the present application, are significantly more complex due to severe boundary reflexes. The increased complexity of multi-resonance plates is caused by a plurality of additional main lobes which are superimposed on the so-called main lobe which has a frequency-dependent direction, thereby producing a strongly fanned-out directional diagram which also has a strong frequency-dependence. Typically, the directional diagrams of the multi-resonance plates to be described here are on average oriented away from the surface normal. This characteristic has the effect that the space plays a much more important role in the projection of the sound waves.
The sound panel is constructed according to a sandwich principle, in that two opposing surfaces of a very light for layer are connected with a thin cover layer, for example through an adhesive bond. For enhancing the sound reproduction characteristic of the sound panel, the material used for the cover layer should have a particularly high dilatational wave velocity.
Suitable materials for the cover layers are, for example, thin metal foils or fiber-reinforced plastic foils.
The core layer also has to meet certain requirements. The materials used for the core layer should have a small mass density and a small damping. In addition, the materials for the cover layer should have a sufficiently high shear modulus perpendicular to the surfaces to which the cover layers are applied. The materials used for the core layers should also have a small elasticity module in the direction in which the core layer formed from this material has its largest expansion. The two latter requirements which initially appear to be contradictory, are most likely satisfied by a core layer that has a hole structure with openings between the two surfaces to which the cover layers are applied. The openings should preferably have a small cross-section. Instead of core layers with the hole structure, rigid expanded foam can also be used as a material for the core layers, since such foams have a suitable shear and elasticity module in spite of their isotropic material properties. It should also be noted that if the core layer is made of rigid expanded foams, then the anisotropic characteristic of the sound panel should be provided by the cover layers.
The sound panels are driven by drivers whichxe2x80x94as described in DE-A-197 57 097xe2x80x94are either attached to or integrated with the respective sound panel.
To reproduce, in particular, low frequency audio signals with the aforedescribed sound panels, specific measures should be taken to reduce acoustic short-circuits. This can be easily realized by providing a sufficiently large sound wall in which the sound panel is installed. However, the sound wall not only increases the cost, but may also reduce the surface area of the sound panel that is potentially available for radiating sound.
It is therefore an object of the invention to provide a sound panel which reduces or eliminates acoustic short-circuits without requiring additional space, and at the same time utilizes the surface area of the sound panel that is a available for radiating the sound, to increase the sound level and/or broaden the frequency range.
When two sound panels are arranged with a mutual spacing A therebetween and surrounded by a frame, a space is formed between the two sound panels and the frame which separates those sound waves that are radiated in this space from the sound waves that are radiated by the opposing surfaces of the sound panels. If the drivers that drive the sound panels arranged with the mutual spacing A are addressed electrically in parallel, then a monopole radiator is formed, which provides a particularly advantageous acoustic irradiation of the auditorium due to the fact that the sound panels oscillate in opposite directions and hence operate according to the principle of a breathing sphere and/or wall.
The sound panels and/or the frame should include sound exit openings if the sound energy that radiates into the space formed by the sound panels and the frame, is to be available for radiating sound into the auditorium. In this case, the space formed between the sound panels and the frame can form a bass reflex volume and the respective sound exit openings a bass reflex opening. The sound exit opening can also be connected to a transmission line.
It should be mentioned that the device according to the invention is not limited to the reproduction of bass frequencies. For example, a combined device can be built, wherein one sound panel together with the aforedescribed space can be used to predominantly reproduce bass frequencies, whereas the other sound panel can be optimized for reproducing, for example, the midrange/high frequency range. The transmission ranges of the two sound panels can be optimized by suitably selecting their respective materials and/or dimensions, for example by choosing a different thickness and/or the material for the core layer and/or the cover layers, respectively. Moreover, the sound panel that reproduces the bass frequencies, unlike the other sound panels, need not directly face a listener, because of the human ear is generally unable to pinpoint the origin of sound with a frequency below 100 Hz.
The driver driving one sound panel can simultaneously provide a counter support for the driver driving the other sound panel. Such an arrangement is particularly advantageous if the connected drivers that drive the different sound panels are electrically connected so that the current flow is codirectional, causing the different sound panels to oscillate in is opposite directions.
The device can be made less complex if the two connected drivers are formed as common drivers. This should be understood as representing an arrangement wherein common components and magnetic circuits contribute to driving both sound panels that are arranged with the spacing A.
The rigidity of the two sound panels can be increased by supporting the two sound panels with resilient elements, e.g. springs, arranged in the gap A. In addition, the resilient elements have damping attributes and can therefore be used to tune the sound panels.
Advantageously, the drivers can be attached to the elements (frame and/or springs) rather than to the sound panels. With the drivers supported in this way, mass loading does not at all or only slightly interfere with the propagation of the bending waves in the sound panels. The drivers also do not contribute to the damping.